In constructing a fluid flow transfer device such as a hemodialyzer that uses a pleated membrane, it is desirable to pot the membrane tips to the interior of the housing on the blood side of the membrane to force the blood down into the membrane folds and thereby prevent shunting of the blood from inlet to outlet without being dialyzed within those folds. Such potting is effectively done with an initially easily flowable material such as low viscosity polyurethane. It is important, however, to keep the potting from flowing into the blood inlet and outlet manifold areas and into the membrane folds opposite those areas, or blood flow into or out of those folds will be undesirably blocked and the dialyzer will have a reduced capacity.